Method for the manufacturing of extruded profiles that can be anodized with high gloss surfaces, the profiles being extruded of an age hardenable aluminium alloy that can be recrystallized after cold deformation, for example a 6xxx or a 7xxx alloy

ABSTRACT

Method for the manufacturing of extruded profiles that can be anodized with high gloss surfaces, the profiles being extruded of an age hardenable aluminium alloy that can be recrystallized after cold deformation, for example a 6xxx or 7xxx alloy, where the alloy initially is cast to extrusion billet(s), where the billets are homogenized at a holding temperature between 480° C. and 620° C. and soaked at this temperature for 0-12 hours, where after the billets are subjected to cooling from the homogenization temperature at a rate of 150° C./h or faster, a) the billets are preheated to a temperature between 400 and 540° C. and extruded preferably to a solid shape profile and cooled rapidly down to room temperature, b) deforming the profile more than 10% by a cold rolling operation, where after c) the profile is flash annealed with a heating time of maximum two minutes to a temperature of between 400-530° C. and held at this temperature for not more than 5 minutes to obtain an average grain size of about 100 μm or less, and subsequently quenched, d) and the profile is finally aged.

The present invention relates to a method for the manufacturing ofextruded profiles that can be anodized with high gloss surfaces, theprofiles being extruded of an age hardenable aluminium alloy that can berecrystallized after cold deformation, for example 6xxx (Al—Mg—Si)alloys or 7xxx (Al—Mg—Zn) alloys.

The oxide layer (Al₂O₃) formed during anodizing is build up bydissolving the outer layer of the aluminium. For each 3 μm of oxidelayer formed 2 μm of the aluminium is dissolved. Since the oxide layeris bulkier than the aluminium the total thickness will then increase by1 μm. In order to obtain high gloss of an anodized aluminium product itis important to keep the amount of constituent particles with a diameterlarger than approximately 0.3 μm (S. Wernick, R. Pinner and P. G.Sheasby, The Surface Treatment and Finishing of Aluminium and itsAlloys, ASM INTERNATIONAL, FINISHING PUBLICATIONS LTD, Fifth Edition Vol1, 1987, p. 143) at a low level, since these particles will be embeddedin the anodized layer and cause a reduction in the gloss. The mostimportant factor to achieve this is to keep the amount of Fe at a lowlevel, since primary AlFeSi particles are insoluble in the aluminiummatrix. Typically, alloys used for high gloss products have a maximumlimit of Fe around 0.12 wt %. Gloss is thus also reduced with increasingthickness of the oxide layer formed during anodizing since moreparticles then will be embedded. Moreover the process parameters usedduring anodizing also affect the gloss.

Hardening precipitates are formed during the artificial ageing process(e.g. β″-MgSi) from the addition of Mg and Si. If Cu is added insufficient amount other phases than β″ may form (e.g. Q′ and L) (CalinD. Marioara, et. al., Improving Thermal Stability in Cu-ContainingAl—Mg—Si Alloys by Precipitate Optimization, METALLURGICAL AND MATERIALSTRANSACTIONS A, March 2014). These hardening precipitates are muchsmaller than 0.3 μm and are therefore not reducing the gloss in the sameway as the primary AlFeSi particles. The strength requirement for thealloy determines the necessary amount of Mg, Si and Cu in the alloy. Inorder to maximize the gloss it is necessary to process the material in away where precipitation of larger non-hardening phases (e.g. β′-MgSi andβ-Mg₂Si) of Mg, Si and Cu is avoided. This is easiest to obtain for 6060and 6063 type of alloys where the Mg and Si contents are relatively low.Higher alloyed material requires higher temperatures in the extrusion orsolutionising processes and faster cooling afterwards to avoidprecipitation of such particles.

Alloying elements such as Mn, Cr, Zr or Sc can be added to formdispersoid particles during homogenization. Frequently, these elementsare added in high amounts in order to prevent recrystallization in theextruded profile. However, it can be beneficial to add these elements insmaller amounts to only have some dispersoid particles in the alloy inorder to avoid grain growth during homogenization and after therecrystallization process occurring in the extrusion process or in aseparate recrystallization and solutionising process for the colddeformed material. The size of these particles is typically between0.01-0.2 μm. Thus, such particles can be added, at least in a relativelow number, without significantly affecting the gloss. However, thenumber of dispersoid particles should not be so high that the exposedareas of the profile surface get a mixture of a non-recrystallized and arecrystallized structure or a fully recrystallized structure with alarge and uneven grain size. Addition of elements that form dispersoidparticles can also give an unwanted color of the anodizing layer, orthey can give an unwanted surface appearance due to a strong texture ofthe recrystallized grains.

If an anodized surface contains large grains the individual grains canbe detected by the naked eye. This surface defect is frequently calledmottling. The best surface appearance is obtained when the average grainsize is smaller than approximately 70 μm and the grains mainly arerandomly orientated.

If the processing of the material is satisfactory there will be no largeβ′-MgSi or β-Mg₂Si particles present in the extruded and aged profilesamples. In such a case the gloss will be more or less proportional tothe amount of Fe in the alloy for a given anodizing process. To maximizethe gloss one would like to minimize the Fe content. Reducing the Fecontent will increase the price of the aluminium since it will be morecostly to produce. It will require alumina with low Fe and lowcontribution of Fe from the anodes. The processing in the electrolysisand the casthouse also has to be adapted in order to produce aluminiumwith very low Fe content. The main problem by using very low Fe contentsis, however, the ability to control the grain size in the billet and inthe extruded profile.

From Japanese patent publication No. 10-306336 is known an aluminiumalloy extruded material having high surface gloss after anodic oxidationtreatment where the surface gloss allegedly is made uniform byspecifying the number of the particles of Mg₂Si participated in thematrix. This is obtained with a specific heat treatment procedure priorto and after extrusion.

With the present invention it is provided a method for the manufacturingof products that can be anodized high gloss surfaces from extrudedprofiles of for example 6xxx or 7xxx alloys, with excellent mechanicalproperties and at low costs.

The method according to the invention is characterized by the featuresas defined in the accompanying independent claim 1. Further embodimentsare defined in the subordinate claims 2-14.

The invention will be further described in the following by way ofexamples and with reference to the drawings and figures where:

FIG. 1 is a photo of a quarter of a macro etched billet slice (ø228 mmin diameter) with abnormal grains,

FIG. 2 light optical micrograph showing a typical grain structure of a6060 alloy through the thickness of a thick solid shape extrudedprofile. The sample is taken from a transverse cross section and isanodized and viewed in polarized light,

FIG. 3 is a principal sketch of an industrial processing line forperforming the cold rolling and the annealing process described in thepresent invention,

FIG. 4 shows light optical micrographs of samples from example 1 showingthe grain structure of a 6060 alloy in the middle of the transversecross section for the as extruded profile and for the samples that werecold rolled to give 10, 20, 40 and 60% reduction in the thickness priorto annealing. All samples are anodized and viewed in polarized light,

FIG. 5 shows grain structure in an as cast billet (ø95 mm diameter)without grain refiner, which was used in example 2 of the presentapplication. Picture of a macro etched billet slice to the left andanodized sample viewed in polarized light in a light optical microscopeto the right,

FIG. 6 are light optical micrographs showing the AlFeSi particles in ahomogenized billet cast without grain refiner (upper picture) and in ahomogenized billet cast with grain refiner (lower picture). The positionof the samples in the billet is approximately half radius,

FIG. 7 is a light optical micrograph of an as extruded sample in example2 of the application, showing the grain structure in a transverse crosssection close to the surface. Anodized and viewed in polarized light,

FIG. 8 shows light optical micrographs of samples from example 2,showing the grain structure in the middle of the transverse crosssection for the as extruded profile and the samples that were coldrolled to give 20, 30, 40 and 50% reduction in the thickness prior toannealing. All samples are anodized and viewed in polarized light,

FIG. 9 shows further light optical micrographs of samples from example 2of the present application, showing the grain structure in the middle ofthe cross section for samples that were cold rolled to 40% reduction inthe thickness prior to annealing in air (upper) and in a salt bath(lower). Both samples are anodized and viewed in polarized light,

FIG. 10 light optical micrograph of a sample of alloy 7030 from example3, showing the grain structure through the transverse cross section of aprofile that has been cold rolled to 10% reduction in thickness andsubsequently flash annealed in a salt bath. The sample is anodized andviewed in polarized light,

FIG. 11 light optical micrographs of samples of alloy 7030 from example3, showing the grain structure in the middle of the transverse crosssection for the as extruded profile and the samples that were coldrolled to give 20, 30, 40 and 50% reduction in the thickness prior toannealing. All samples are anodized and viewed in polarized light,

FIG. 12 the grain structure towards one end of the transverse profilecross section of a 40% cold rolled and annealed sample of alloy 7030.The sample is anodized and viewed in polarized light.

When the Fe content is below approximately 0.10 wt % in a 6060 or 6063type of alloy the chance of getting abnormal grains (grains that growand consume other grains that were formed during casting) in the billetduring homogenization becomes very high. Therefore, a grain size ofseveral centimeters is very common in billets of alloys with very lowamounts of Fe. An example of abnormal grains in a homogenized billetwith low Fe content is shown in FIG. 1.

For a 7xxx alloy the homogenizing temperatures are normally lower thanfor a 6060 alloy. This may reduce the problem with abnormal grain growthduring homogenization.

The billet grain size will probably not affect the grain size in theextruded profile much if the extent of deformation is high, for examplewhen extruding thin walled hollow profiles. For solid shapes, andespecially for thick walled profiles, the billet grain size will mostlikely affect the grain size in the extruded profile. An additionalchallenge is that the billet temperature needs to be rather high inorder to dissolve the Mg₂Si particles, and a high billet temperaturemakes it more difficult to obtain a small grain size after extrusion.

In an extruded profile, one usually sees a surface layer of mainlyrandomly oriented grains and typically one or a few grains in thickness.Underneath this layer one typically finds a region of larger grains. Thethickness of this layer varies, and is usually thicker for a thickwalled and wide solid shape profile and thicker towards the back end ofthe extruded length. An example of a typical grain structure in a crosssection of a thick walled industrially extruded profile can be seen inFIG. 2. Below the layer of larger grains the grain structure istypically more homogeneous. The grains in the homogeneous center regionof the cross section are predominantly aligned in one direction, with astrong cube texture. This is often seen in a micrograph of the grainstructure in the cross section by small differences in the color of thegrains.

More and more consumer electronics like mobile phones, tablets and laptops are made of aluminium from extruded profiles. If the profilesurface could have been used without any machining the grain structurein the anodized surface would probably be okay in most cases. However,very often there is a need to machine the extruded profile to make theshape and the dimensional tolerances of the final product. In that casethe exposed surface can consist of grains from the coarse grain layerbeneath the surface layer of the extruded profile. Due to this theentire coarse grain layer has to be removed before starting to machinethe shape of the final product. The thickness of the layer that has tobe removed due to coarse grains will vary with the width of the profileand the extrusion conditions and is typically in the range of 0.2 to 1mm.

The present invention deals with the task to get a homogeneous grainstructure with an average grain size below approximately 70 μmirrespective of the Fe content, the grain size in the billet prior toextrusion and the extrusion conditions.

Solid shape profiles which are blanks for consumer electronics will bemore or less flat, but could possibly have some features in the crosssection in order to save material and machining. Such profiles aretherefore very well suited for cold rolling after extrusion. By coldrolling a profile by a minimum of 10% followed by flash annealing a newrecrystallization process will take place. With sufficient deformationand a proper annealing process the resulting grain structure will behomogeneous over the cross section with a much more random orientationof the grains than in the as extruded profile. The grain size will inaddition to the alloy content, depend on the degree of cold deformation,the annealing temperature, the heat up conditions and the time at theannealing temperature. In an alloy with very low Fe and no dispersoidparticles the recrystallization will take place at a low temperature,most likely during heating to the annealing temperature. One issue willthen be to avoid grain growth at the annealing temperature when thereare almost no particles in the material to pin the grains.

The annealing temperature should preferably be above the solvustemperature for Mg₂Si particles (6xxx) or Zn₂Mg particles (7xxx) inorder to avoid particles that can reduce the strength and the gloss ofthe anodized material. In addition, the time at this annealingtemperature should be as short as possible in order to avoid graingrowth. Therefore, the material should be processed through extrusion ina way that Mg₂Si or Zn₂Mg particles are avoided. This means sufficientlyhigh billet temperature in combination with a high enough exittemperature from extrusion and fast cooling of the profile afterextrusion. With no Mg₂Si or Zn₂Mg particles in the material prior tocold rolling and annealing there is no need for a holding time for thematerial at the annealing temperature.

The consequence of annealing at temperatures below the solvustemperature will be that Mg—Si containing precipitates or Zn₂Mgprecipitates larger than approximately 0.3 μm may form. These particleswill contribute to a reduction in the gloss and in the strength of thematerial. The amount of this reduction will depend on the actualtime-temperature history during the flash annealing and coolingoperation and the composition of the alloy.

An industrial process to perform the cold rolling and the annealingprocess could be done as shown schematically in FIG. 3. The cold rollingstation should be followed by a station for performing fast heating tothe annealing temperature. Using induction heating is probably the bestway to do this. With enough power and induction coils that fit the shapeof the profile and good process control, it should be possible to heatthe material to a temperature around 500° C. (depending on thecomposition and thereby the solvus temperature of the alloy) within avery short time and with sufficient accuracy in temperature.

In order to avoid precipitation of Mg—Si containing precipitates orZn₂Mg precipitates larger than approximately 0.3 μm the profile needs tobe cooled rather rapidly down to room temperature. The reason for thisis described in a previous section. Thus, preferably according to thepresent invention, the profile is flash annealed with a heating time ofmaximum two minutes to a temperature of between 450-530° C. and held atthis temperature for not more than 5 minutes and subsequently quenched.

After the annealing operation there is probably a need to removeresidual stresses from the quenching operation. The best way to thiswould probably be to stretch the material in way similar to what is doneafter extrusion.

After the annealing process the final ageing of the material can forexample be done with the patented dual rate ageing cycle (U. Tundal andO. Reiso, EP 1 155 161 B1) to get maximum strength with minimum amountof alloying elements.

The invention will be further described in the following by way ofexamples.

EXAMPLE 1

Billets with diameter 95 mm were cast in a lab casting facility usingthe Hycast hot-top gas-slip technology (as described in EP 0 778 097 B1)and a TiB₂ based grain refiner. The composition of the alloy is shown inTable 1.

TABLE 1 Chemical composition of the alloy used in example 1 Mg Si Fe MnCr Cu Zn Zr Ti B Al 0.354 0.539 0.110 0.001 0.001 0.001 0.002 0.0010.012 0.002 98.95

The billets were homogenized at 575° C. for 2 hours and 15 minutesfollowed by cooling at a rate of approximately 400° C. per hour.Extrusion of the billets was performed at an 8 MN laboratory extrusionpress with a 100 mm diameter container to a profile with 5×40 mm² crosssection. The billet preheating temperature was approximately 500° C. andthe extrusion speed 20 m/min. After extrusion the profile was quenchedin water.

A 50 cm long piece from the front part of the extruded profile was coldrolled to give 10, 20, 40 and 60% reduction in the thickness. Thesamples that were cold rolled to different thicknesses were thenannealed in a salt bath which had been preheated to 500° C. A hole wasdrilled into each of the samples to fit a thermocouple. The heating timeto temperature was in the range 5-10 seconds, depending on the thicknessof the sample. When a sample was put into the salt bath a holding timeof 10 seconds started when the temperature reached 490° C. Afterannealing the samples were quenched in water.

The cross section of all samples (in all examples) were prepared bygrinding and mechanical polishing with a final step using 1 μm diamondpaste. In order to make the grains visible in polarised light, anodisingwas performed in a Struers Lectropol-5 with the following parameters.Voltage: 45 V; Flow rate: 3; Temperature: −5° C.; Time: 2 minutes. Theelectrolyte had the following ingredients: 74% distilled water; 24%ethanol; 1% HBF₄ (35%); 1% HF (40%).

Prior to extrusion the billets had an even and small grain size. The asextruded sample in FIG. 4 shows a homogeneous grain size throughout thecross section. In this case there is no significant coarse grain layerbelow the surface. This is maybe because the sample is smaller than thesample shown in FIG. 2 and maybe also because it is taken from the frontpart of the extruded length. It is evident that the grains under therandomly oriented layer of grains in the profile surface area arepredominantly aligned in one direction since the color contrast betweenthe grains is low.

As can be seen from the large color contrast, the cold rolled andannealed samples show a much more random orientation of the grains thanthe as extruded sample. This confirms that these samples are fullyrecrystallized after annealing. The samples that were cold rolled to 10and 20% reduction in thicknesses clearly have an uneven grain structurewith the largest grains in the middle of the cross section. The samplesthat were cold rolled to 40 and 60% reduction in thicknesses have aneven grain structure throughout the cross section. The grain sizes ofthe samples shown in FIG. 4 (measured 250 μm below the surface of thecross sections) are shown in Table 2.

TABLE 2 Average grain sizes of the 6060 alloy samples in example 1 asmeasured 250 μm below the surface of the cross section. The as extrudedgrain size is very uncertain due to the very low contrast between theindividual grains. 10% cold 20% cold 40% cold 60% cold rolled + rolled +rolled + rolled + As extruded annealed annealed annealed annealed ~87 μm79 μm 60 μm 44 μm 33 μm

EXAMPLE 2

Billets with diameter 95 mm were cast in a lab casting facility usingthe Hycast hot-top gas-slip technology without using a grain refiner. Apicture of a macro etched billet slice is shown in FIG. 5 together witha micrograph showing an anodized sample viewed in polarized light in thelight optical microscope. Towards the surface there are some relativelylarge equiaxed grains, but a large part of the cross section of thebillet slice consists of feather crystals. The composition of the alloyis shown in Table 3.

TABLE 3 Chemical composition of the alloy used in example 2 Mg Si Fe MnCr Cu Zn Zr Ti B Al 0.380 0.473 0.092 0.002 0.001 0.001 0.006 0.0000.004 0.000 99.00

The cast billets were homogenized at 575° C. for 2 hours and 15 minutesfollowed by cooling at a rate of approximately 400° C. per hour.Micrographs of the particle structure in the billets from the twodifferent alloys in examples 1 and 2 are shown in FIG. 6. The materialcast without grain refiner (upper picture) shows Fe containing particles(mainly α-AlFeSi) that are smaller and much more evenly distributed thanthe Fe containing particles (mainly β-AlFeSi) in material cast withgrain refiner (lower picture). In the latter case the AlFeSi particlesmainly are located at the grain boundaries. In both cases the Fe/Siratio is very low, which makes β-AlFeSi particles very stable in thehomogenizing process. A particle structure as shown in the material castwithout a grain refiner would be beneficial in avoiding alignment ofparticles and possible visible dark lines in the extruded and anodizedhigh gloss surface.

The billets were extruded at an 8 MN laboratory extrusion press with a100 mm diameter container to a profile with a cross section of 5×40 mm².The billet preheating temperature was approximately 500° C. and theextrusion speed 20 m/min. After extrusion the profile was quenched inwater.

A 100 cm long piece from the back part of the extruded profile was coldrolled to give 20, 30, 40 and 50% reduction in the thickness. Thesamples that were cold rolled to different thicknesses were thenannealed in a salt bath which had been preheated to 500° C. A hole wasdrilled into each of the samples to fit a thermocouple. When a samplewas put into the salt bath the holding time of 10 seconds started whenthe temperature reached 490° C. After annealing the samples werequenched in water. In addition one sample of the material cold rolled to40% reduction in thickness was held 5 minutes at 500° C. Yet anothersample of the material cold rolled to 40% reduction in thickness washeated in an air circulating oven at a considerably lower heating rateto the annealing temperature than that obtained in a salt bath.

A micrograph of the as extruded sample is shown in FIG. 7. It seems likesome of the grains below the surface are considerably larger than 100μm, which could give some unwanted effects in the surface appearance.Inside the surface region the grains are strongly aligned in onedirection, which gives very little contrast between each individualgrain in the micrograph.

FIG. 8 shows micrographs of the grain structure in the as extrudedsample as well as samples that have been cold rolled 20, 30, 40 and 50%and thereafter annealed. As also seen in example 1, one can see from thelarge color contrast that the cold rolled and annealed samples show amuch more random orientation of the grains than the as extruded sample.The sample that was cold rolled to 20% reduction in thickness clearlyhas an uneven grain structure with the largest grains in the middle ofthe cross section. The sample cold rolled to 30% reduction in thicknesshas smaller grains and a more even grain structure, but the grains inthe middle still are somewhat larger than those towards the surfaces.The samples that were cold rolled to 40 and 50% reduction in thicknesseshave a smaller grain size and an even grain structure throughout thecross section. As also shown in Table 4 (below) the grain size seems tobe similar for the samples cold rolled to 40 and 50% reduction inthicknesses.

TABLE 4 Average grain sizes of the 6060 samples in example 2 as measured250 μm below the surface of the cross section. The as extruded grainsize is very uncertain due to the very low contrast between theindividual grains. 20% cold 30% cold 40% cold 50% cold rolled + rolled +rolled + rolled + As extruded annealed annealed annealed annealed ~88 μm101 μm 95 μm 52 μm 57 μm

The sample that was cold rolled to 40% reduction in thickness and heldat 500° C. for 5 minutes did not show any grain growth. The reason forthis is probably that the number of AlFeSi-particles is high enough toprevent grain growth. With even lower Fe contents than 0.09 wt % aholding time of 5 minutes at this temperature could cause grain growthin the sample.

FIG. 9 shows that the sample heated in an air-circulating furnace (6-7minutes heating time) has a more uneven grain structure and a slightlylarger grain size than the sample that was rapidly heated (5-10 seconds)in a salt bath up to the solutionizing temperature. The reason for thisis probably linked to precipitation of Mg—Si particles at the grainboundaries, which are pinning the nuclei for new grains during the heatup process. In a sample which is slowly heated in air there is enoughtime for precipitation of Mg—Si particles to prevent the nuclei for newgrains from growing until the particles start to dissolve again, i.e.when the sample is approaching the solvus temperature of the alloy. Inthis process some grains will probably start to grow earlier than othersand therefore get larger, resulting in an uneven grain structure whenthe recrystallization process is complete.

Example 2 shows that it is beneficial to heat the cold rolled samplefast to the solutionizing temperature to obtain an even grain size andthat a holding time of only 10 seconds is sufficient to obtain a fullyrecrystallized grain structure.

Example 2 also shows that the final grain structure in the blanks couldbe perfect for providing attractive high gloss anodized surfaces eventhough the billet grain structure is regarded as being far from optimumwhen it is cast without grain refiner.

EXAMPLE 3

Billets with diameter 95 mm of a 7030 alloy were cast in a lab castingfacility using the Hycast hot-top gas-slip technology and a TiB₂ basedgrain refiner. The chemical composition of the alloy is shown in Table5.

TABLE 5 Chemical composition of the alloy used in example 3 Mg Si Fe MnCr Cu Zn Ti Al 1.152 0.070 0.094 0.000 0.000 0.206 4.964 0.006 93.71

The billets of the 7030 alloy were homogenised for 4 hours at 500° C.The billets were extruded at an 8 MN laboratory extrusion press with a100 mm diameter container to a profile with a cross section of 5×40 mm².The billet preheating temperature was approximately 500° C. and theextrusion speed 12.5 m/min. After extrusion the profile was quenched inwater.

A 100 cm long piece from the extruded profile was cold rolled to give20, 30, 40 and 50% reduction in the thickness.

The cold rolled samples at different thicknesses were then, one by one,put into a salt bath that had been preheated to 500° C. With athermocouple drilled into each sample it was possible to monitor thetemperature of the sample. All samples were held approximately 10seconds at a temperature above 495° C. before quenching in water. Theheating rates of the samples depended on the thickness, but in all casesthe heating time was less than 10 seconds.

FIG. 10 shows the grain structure through the transverse cross sectionof a 7030 sample that has been cold rolled to a 10% reduction inthickness and subsequently flash annealed in a salt bath. As can beseen, the grain structure is very uneven, with some grains being morethan 500 μm in diameter. This shows that 10% deformation by rolling istoo little to create a uniform grain structure through the cross sectionof the material.

FIG. 11 shows the grain structures through the thickness of a transversecross section of an as extruded 7030 profile as well as of samples thathave been cold rolled to 20, 30, 40 and 50% reduction in thicknesses andsubsequently flash annealed. The grain structure of the as extrudedsample is significantly coarser than the grain structure in the 6060alloy. This could either be a result of the lower extrusion speed usedfor the 7030 alloy or a higher solute drag from the high amount of Mg,Zn and Cu in this alloy. The sample rolled 20% show a slightly coarsergrain structure than the as extruded sample, especially in the middle ofthe cross section. The sample rolled 30% has a grain structure thatwould fulfil the requirements of a grain size below about 70 μm, but thegrain size in the middle is somewhat larger than towards the surface.The grain structures in the samples cold rolled by 40 and 50% show avery nice grain structure throughout the cross section. Based on thevisual appearance of the anodized grain structures, the as extrudedsample of the 7030 alloy does not seem to have the same strong cubetexture as the 6060 alloy.

As shown in FIG. 12, the grain structure is also very uniform towardsthe ends of the cross section when a sample of a 7030 alloy has beencold rolled by 40% before the flash annealing process.

The grain sizes of some of the samples depend on the depth below thesurface of the cross section. Some samples have very coarse grains inthe middle of the cross section and finer grains towards both surfaces.A typical machining depth to remove the coarse surface grain layer in asmall profile like this would be around 250 μm, and this depth waschosen for the grain size measurements. In Table 6 (below) the grainsizes for the alloy of Example 3 are listed. By looking at these grainsize measurements alone, all the samples seem to fulfil the requirementof a grain size below approximately 70 μm.

However, the pictures in FIG. 11 give a better overview of the grainstructures in the samples. From the grain size measurements and thepictures one can state that all samples with 30, 40 and 50% cold rollingfollowed by annealing fulfil the grain structure requirements for thealloy. With less deformation the grain structure seems to be too uneven.

TABLE 6 Average grain sizes of the 7030 alloy samples in example 3 asmeasured 250 μm below the surface of the transverse cross section. 20%cold 30% cold 40% cold 50% cold rolled + rolled + rolled + rolled + Asextruded annealed annealed annealed annealed 87 μm 65 μm 45 μm 37 μm 31μm

The main benefit of the present invention is that it is possible toobtain a grain structure with an even grain size and a close to randomtexture throughout the cross section of a profile irrespective of thegrain size in the profile after it has been extruded and thus alsoirrespective of the grain structure of the billet before extrusion. Thisimprovement in grain structure is obtained by cold rolling deformationof the extruded profile followed by flash annealing.

An extruded thick walled flat profile will in most cases have a coarsegrain layer that according to the state of the art has to be removed inorder to obtain a smooth anodized surface with a minimum of defects inthe final product. The amount of material that would have to be removedin the as extruded cross section is typically in the range 7-15%.

Moreover, the cold rolling will ensure a very accurate thickness andflatness of the profile, and for that reason considerably reduce theneed for machining. An extruded profile will have much more variation inthe thickness, typically ±0.15 mm (the variation could also be higherfor very wide profiles, especially for 7xxx alloys).

Since the grain size in the billet and the extruded profile is of littleimportance for the resulting grain size in the cold rolled and annealedblanks there is a possibility of casting the billets with a minimum oreven completely without the use of a grain refiner. In order to avoidcenter cracks in the billets in the startup of the cast it could bebeneficial to add some grain refiner in the first metal to cast. Thegrain refiner itself could be a source for inclusions that can causefailures in the anodized surface. Another benefit of not using a grainrefiner is that the melt cleaning with the use of ceramic foam filterswill be more effective on other type of inclusions (Nicholas Towsey,Wolfgang Schneider and Hans-Peter Krug, A comprehensive study of ceramicfoam filtration, 7th Australasian Asian Pacific Course & Conference,Aluminium Cast House Technology: Theory & Practice, P. Whiteley and J.Grandfield (TMS: 2001)

The possibility of reducing the Fe content and still obtain an adequategrain structure will significantly improve with the use of the presentinvention. The lower Fe content can either be used to improve the gloss,or to keep the current gloss but add a thicker and more wear resistantoxide layer to the anodized product. The latter will make the productmore durable.

Even though there is extra cost associated with the cold rolling andannealing process to obtain the uniform and random grain structure, thiswill probably be more than compensated for by the savings due to reducedmachining and reduced material consumption.

1. Method for the manufacturing of extruded profiles that can beanodized with high gloss surfaces, the profiles being extruded of an agehardenable aluminium alloy that can be recrystallized after colddeformation, for example a 6xxx or a 7xxx alloy, where the alloyinitially is cast to extrusion billet(s), where the billets arehomogenized at a holding temperature between 480° C. and 620° C. andsoaked at this temperature for 0-12 hours, where after the billets aresubjected to cooling from the homogenization temperature at a rate of150° C./h or faster, a) the billets are preheated to a temperaturebetween 400 and 540° C. and extruded preferably to a solid shape profileand cooled rapidly down to room temperature, b) deforming the profilemore than 10% by a cold rolling operation, where after c) the profile isflash annealed with a heating time of maximum two minutes to atemperature of between 400-530° C. and held at this temperature for notmore than 5 minutes to obtain an average grain size of about 100 μm orless, and subsequently quenched, and d) the profile is finally aged. 2.Method according to claim 1 characterised in that the profile betweenstep c) and d) is further subjected to a cold deforming operation,preferably by stretching to remove residual stresses from cooling. 3.Method according to claim 1 characterised in that the profile betweenstep c) and d) is cut into blanks that is cold formed to a shape thatsaves material and machining time to produce the final product. 4.Method according to claim 1 characterised in that the alloy iscontaining in wt. %: Si: 0.00-1.00 Mg: 0.25-2.00 Fe: 0.00-0.25 Cu:0.00-0.50 Mn: 0.00-0.20 Cr: 0.00-0.10 Zr: 0.00-0.20 Sc: 0.00-0.10 Zn:0.00-7.00 Ti: 0.00-0.05, and including incidental impurities and balanceAl.
 5. Method according to claim 1 characterised in that the alloy is a7xxx alloy containing in wt. %: Si: 0.00-0.30 Mg: 0.50-2.00 Fe:0.00-0.15 Cu: 0.00-0.30 Mn: 0.00-0.20 Cr: 0.00-0.10 Zr: 0.00-0.20 Sc:0.00-0.10 Zn: 3.00-7.00 Ti: 0.00-0.05, and including incidentalimpurities and balance Al.
 6. Method according to claim 1 characterisedin that the alloy is a 6xxx alloy containing in wt. %: Si: 0.30-1.00 Mg:0.25-1.00 Fe: 0.00-0.25 Cu: 0.00-0.50 Mn: 0.00-0.20 Cr: 0.00-0.10 Zr:0.00-0.20 Sc: 0.00-0.10 Zn: 0.00-0.50 Ti: 0.00-0.05, and includingincidental impurities and balance Al.
 7. Method according to claim 1,characterised in that, the profile according to step b) is deformed morethan 20%.
 8. Method according to claim 1, characterised in that theprofile according to step b) preferably is deformed between 30 and 50%.9. Method according to claim 1, characterised in that the profile isflash annealed according to step c) with a heating time of maximum 20seconds to a temperature between 400-530° C. and held at thistemperature for not more than 1 minute.
 10. Method according to claim 1,characterised in that the flash anneal heating according to step c) isobtained by induction heating of the profile.
 11. Method according toclaim 1, characterised in that the flash anneal heating according tostep c) is obtained by subjecting the profile to a salt bath or otherconvection or radiation heating means providing high heating rates. 12.Method according to claim 1, characterised in that the alloy is castwithout the use of grain refiner, except in the start-up of the castingoperation.
 13. Method according to claim 1, characterised in that theageing, step d) is a one step, two step or a dual rate ageing operationto a final hold temperature between 100° C. and 220° C. and where thetotal ageing cycle is performed in a time span of between 3 and 24hours.
 14. Method according to claim 1, characterised in that theaverage grain size according to process step c) is 70 μm or less.